For many years it has been common practice in the metal working art to use flame or plasma torches to cut out large sections of plate. Such torches can be held in the hand of an operator and guided by him; they can be mounted on tracks and provided with motors for straight line cutting; they can be pivoted about a central point to cut out circles; they can be adapted to follow a template cut out in advance; and they can be used on machines provided with optical-electronic "tracers" and appropriate servomotors and controls for movement of the torch (or torches) according to a blueprint or other plan. The invention relates to the last class of devices. Several different types of tracers have been made which have been of some success. One such is the "circular scan" tracer, an improvement on which is the subject of the present application. Such circular scanning tracer devices are described at least as early as U.S. Pat. No. 3,004,166; more advanced versions of such tracers are described in U.S. Pat. No. 3,704,372. The present invention may to some extent be construed as an improvement on that shown in U.S. Pat. No.3,704,372 and the specification of that patent is therefore hereby adopted and made a part of the present specification. Where possible reference will be made to such patent in order to simplify the description of the present invention.
One deficiency of the tracer shown in U.S. Pat. No. 3,704,372 is that no provision is made for ready adjustment of the "lead" of the tracer. The lead of such a tracer is the distance between the point at which a change in the line being traced is detected and the point on the line corresponding to the actual position of the torch. Some lead is necessary because there is an inevitable delay between the tracer's sensing that a change of direction of travel is necessary and the actual change of direction, mostly due to the momentum of the machine. Momentum being a function of velocity, the lead must be varied with the travel speed. If too little lead is used the torch tends to overshoot corners, particularly if they are sharp, and if too much lead is used corners tend to be rounded off. As the same tracers and cutting machines are commonly used with oxy-fuel cutting torches, for which travel speeds of approximately 30 to 100 ipm (inches per minute) (75-250 cm/min) are appropriate, and with plasma torches which cut at 200 to 250 ipm (500-625 cm/min), a tracer the lead of which is readily adjustable is clearly desirable.
U.S. Pat. No. 3,704,372, referred to above, describes a tracer having means for varying the lead; however, said means are not convenient to use as they require at least partial disassembly of the tracing unit and replacement of one of its parts with another, in order that the lead may be varied. It is therefore, clearly desirable to produce a circular scan tracer having its lead readily adjustable; preferably the lead would be adjustable even while the machine is running which would simplify the adjustment of the lead under given operating conditions.